The goal of this research project is to define the logic by which the brain organizes different types of memories among its component neurons. The project contrasts how the brain organizes olfactory memories learned in association with a rewarding cue and those learned in association with an aversive cue, and delves into some of the underlying mechanisms. The research project utilizes the model system Drosophila melanogaster because of the ease in conditioning the fly using olfactory cues and because of the ability to peer into the brain of living animals and watch the activity of different sets of neurons. The latter approach, functional optical cellular imaging, employs flies carrying transgenes expressing reporters for calcium influx, synaptic transmission, or other neuronal events, to monitor changes in neuronal response properties among the expressing neurons before and after conditioning. To date, six different cellular memory traces have been defined using aversive olfactory conditioning. These traces form in different sets of neurons in the olfactory nervous system and occur with differing temporal dynamics. The memory traces that occur within these same neurons after a rewarding olfactory conditioning event will be examined along with the molecular mechanisms underlying memory trace formation. Since nearly every neuropsychiatric disorder affects memory formation, these studies will aid in understanding memory formation in the normal brain as well as in the diseased brain.